Liquid Crystal Display and Global Dimming Control Method Thereof

1. A global dimming control method of a liquid crystal display for controlling a luminance of backlight depending on a displayed image, the global dimming control method comprising: calculating an average picture level (APL) of the displayed image; calculating the number of pixels of a chromatic region belonging to the displayed image when the APL is equal to or less than a previously determined first threshold value; comparing the number of pixels of the chromatic region with a previously determined second threshold value, calculating a gain value of the chromatic region when the number of pixels of the chromatic region is greater than the second threshold value, and multiplying the gain value by the APL to calculate an adjustment dimming control signal; and controlling the luminance of the backlight using a dimming value obtained based on the adjustment dimming control signal wherein the first threshold value indicates a maximum value of the APL in which a dimming value can be adjusted through global dimming control, and the second threshold value indicates the minimum number of pixels of the chromatic region.

2. The global dimming control method of the liquid crystal display of claim 1 , wherein the calculating of the number of pixels of the chromatic region includes: obtaining a maximum gray level and a minimum gray level in each pixel; multiplying the minimum gray level by a previously determined value for deciding the chromatic region to generate a multiplication value; and comparing the maximum gray level with the multiplication value in each pixel, counting the pixels, in which the maximum gray level is greater than the multiplication value, and outputting a count value as the number of pixels of the chromatic region.

3. The global dimming control method of the liquid crystal display of claim 1 , wherein the gain value is obtained by dividing the number of pixels of the chromatic region by the total number of pixels and adding a quotient of the division to a previously determined weight value.

4. The global dimming control method of the liquid crystal display of claim 3 , wherein the weight value is equal to or greater than 1, and the adjustment dimming control signal is equal to or greater than the APL.

5. The global dimming control method of the liquid crystal display of claim 1 , further comprising, when the number of pixels of the chromatic region is equal to or less than the second threshold value, controlling the luminance of the backlight using a dimming value obtained based on the APL.

6. A liquid crystal display comprising: a liquid crystal display panel; a backlight unit including a plurality of light sources, the backlight unit irradiating light onto a back surface of the liquid crystal display panel; and a global dimming control circuit configured to control a luminance of backlight depending on an image displayed on the liquid crystal display panel, the global dimming control circuit including: an average picture level (APL) calculating unit configured to calculate an APL of the displayed image; a chromatic region size detecting unit configured to calculate the number of pixels of a chromatic region belonging to the displayed image when the APL is equal to or less than a previously determined first threshold value; an adjustment dimming control signal calculating unit configured to find a gain value of the chromatic region, multiply the gain value by the APL, and calculate an adjustment dimming control signal; a selection signal generating unit configured to compare the number of pixels of the chromatic region with a previously determined second threshold value and output a selection signal of a first logic level when the number of pixels of the chromatic region is greater than the second threshold value; a selecting unit configured to output the adjustment dimming control signal in response to the selection signal of the first logic level; and a dimming value generating unit configured to output a dimming value for controlling the luminance of the backlight based on the adjustment dimming control signal; wherein the first threshold value indicates a maximum value of the APL in which a dimming value can be adjusted through global dimming control, and the second threshold value indicates the minimum number of pixels of the chromatic region.

7. The liquid crystal display of claim 6 , wherein the chromatic region size detecting unit includes: a MIN/MAX obtaining unit configured to obtain a maximum gray level and a minimum gray level in each pixel; a multiplication unit configured to multiply the minimum gray level by a previously determined value for deciding the chromatic region and generate a multiplication value; a comparator configured to compare the maximum gray level with the multiplication value in each pixel; and a counter configured to count the pixels, in which the maximum gray level is greater than the multiplication value, and output a count value as the number of pixels of the chromatic region.

8. The liquid crystal display of claim 6 , wherein the gain value is obtained by dividing the number of pixels of the chromatic region by the total number of pixels and adding a quotient of the division to a previously determined weight value.

9. The liquid crystal display of claim 8 , wherein the weight value is equal to or greater than 1, and the adjustment dimming control signal is equal to or greater than the APL.

10. The liquid crystal display of claim 6 , wherein the selection signal generating unit outputs the selection signal of a second logic level when the number of pixels of the chromatic region is equal to or less than the second threshold value, wherein the selecting unit outputs the APL in response to the selection signal of the second logic level, wherein the dimming value generating unit outputs a dimming value for controlling the luminance of the backlight based on the APL.